In order that a liquid crystal display device having an excellent viewing angle characteristic is achieved, an In Plane Switching (IPS) mode instead of a Twisted Nematic (TN) mode has been proposed. In the IPS mode, liquid crystal molecules are controlled with use of a transverse electric field. Meanwhile, in the TN mode, liquid crystal molecules are controlled with use of a longitudinal electric field. Note, however, that in the IPS mode, light is unfortunately used with lower efficiency than in the TN mode.
Under the circumstances, in order to use light with higher efficiency in the IPS mode, Patent Literature 1 has proposed a Fringe Field Switching (FFS) mode. In the FFS mode, liquid crystal molecules are controlled by (i) providing, on one of a pair of substrates, between which a liquid crystal layer is provided, an upper electrode, a lower electrode, and an insulating layer that are provided in a stack so that the upper electrode and the lower electrode overlap each other via the insulating layer, and (ii) using an electric field that is obliquely generated between the upper electrode and the lower electrode (so-called fringe electric field) in response to application of a voltage across the upper electrode and the lower electrode.
In the FFS mode, the upper electrode and the lower electrode are each made of a light-transmitting electroconductive film such as an ITO, and liquid crystals above the upper electrode can be controlled. This allows a higher optical transmittance to be achieved in the FFS mode than in the IPS mode. Furthermore, the FFS mode has an advantage of being great in electric field intensity and allowing a reduction in driving voltage as compared to the IPS mode.
However, in the FFS mode, which is great in electric field intensity, an afterimage (image sticking) phenomenon is more noticeable than in the IPS mode. The afterimage phenomenon is a phenomenon such that a first image that has been displayed by a liquid crystal display device for a long time affects a second image that is displayed by the liquid crystal display device subsequently to the first image. The afterimage phenomenon contributes to deterioration in display performance.
Such an afterimage phenomenon can be reduced by the following proposed method. For example, Patent Literature 2 proposes a method of reducing the afterimage phenomenon by (i) providing one of a pair of substrates, between which a liquid crystal layer is provided, with a group of electrodes, an insulating film, and an alignment film, and (ii) setting the sum of the thickness of the insulating film and the thickness of the alignment film at not less than 0.5 μm and not more than 3 μm.
Furthermore, in transverse electric field modes (e.g., the IPS mode and the FFS mode), in each of which liquid crystal molecules are controlled by causing the liquid crystal molecules to rotate in a plane substantially parallel to a surface of a substrate, not only an afterimage phenomenon that is seen in the TN mode and caused by remaining electric charges but also an afterimage phenomenon unique to the transverse electric field mode occurs. It is considered that the afterimage phenomenon unique to the transverse electric field mode occurs because a surface of an alignment film which controls alignment of liquid crystal molecules is elastically deformed or plastically deformed by a rotation torque caused by twist deformation of liquid crystal molecules, and the elastic deformation or plastic deformation of the surface of the alignment film appears as an afterimage (image sticking).
The afterimage phenomenon unique to the transverse electric field mode can be reduced by (i) using an alignment film having a high elastic modulus and (ii) causing an electric field to be less concentrated. Patent Literatures 3 and 4 each propose a specific method for reducing the afterimage phenomenon unique to the transverse electric field mode.
For example, Patent Literature 3 proposes a method of reducing the afterimage phenomenon by increasing an elastic modulus of an alignment film so that a rotation torque caused by twist deformation of liquid crystal molecules less affects the alignment film.
Furthermore, Patent Literature 4 proposes a method of reducing the afterimage phenomenon, while causing an electric field to be less concentrated, by providing an upper insulating layer between an upper electrode and an alignment film and causing the upper insulating layer to have a higher dielectric constant than an insulating layer provided between the upper electrode and a lower electrode.